Process of concentrating nitric acid



Sept. 20, 1932. 1. J. cox ET AL 1,878,676

PROCESS OF CONCENTRATING NITRIC ACI D Filed Sept. 15, 1928 7 8 M03 Feed H2504- Hui fazd Ail 9 u Smam 10 1.1 Cox Wli/(msr Inveniors GA 14 000502 B1 Hmir aHorn Patented Sept. 20, 1932 UNITED STATES PATENT OFFICE IRVING J. COX AND WILLIAM E. KIRST, OF WOODB'URY, NEW JERSEY, AND CLIFFORD A. WOODIBURY, 0]? .MEDIA, PENNSYLVANIA, ASSIGNORS TO E. I. DU PONT' DE NE- MOURS 80 COMPANY, OF WILMINGTON, DELAWARE, A CORPORATION OF DELAWARE IPROGESS OF OONCENTRATING NITRIC ACID Application filed September 15, 1928. Serial No. 306,164.

This invention'relates to the concentration of nitric acid to a high strength by means of a dehydrating agent, and more particularly to a method of nitric acid concentration wherein the heat of dilution of the dehydrating agent is utilized and the amount ofartificial heat required is reduced to a m1n1- mum.

In order to concentrate nitric acid to at least 90% from lower strengths, say from to 70% HNO it is well known in the art that to economically carry out such a processit is necessary to mix the nitric acid with a dehydrating agent and then apply heat which distills out the nitric acid while the water is retained by the dehydrating agent.

It is common practice to mix sulphuric acid with nitric acid and place the mixture in a retort and distill out the nitric acid by means of heat. The first portion of the acid which distills is strong, that is above 85%,

- and as the distillation proceeds the distillate becomes weaker until the last portions are no stronger than the nitric acid whlch was trio acid and strong sulphuric are run down a packed column in admixture or separately but concurrently, while steam is blown into the bottom of the tower, ascending countercurrent to the liquid acid mixture, and condenses in it and distills out the nitric acid which leaves the top ofthe tower as vapors of highly concentrated nitric acid. Various modifications have been proposed? by the same inventor but in every case the heating medium is steam, which condenses in the acid 7 mixture.

Steam, when used in this manner, possesses a decided disadvantage in that the water resulting from its condensation dilutes the sulphuric acid used in the process and must be boiled out of the sulphuric acid before the acid can be used again.

This invention has as an object the concentration of nitric acid to a high strength by the utilization of the heat of dilution of a dehydrating agent with the nitric acid, thus minimizing the amount of steam or other source of heat required, and furthermore minimizing the dilution of the sulphuric acid by the condensation of steam.

A further object of our invention is a process which may be run in direct combination with a sulphuric acid concentration unit, wherein the hot sulphuric acid produced therein may be utilized without cooling and losing the heat as is the common practice in acid manufacture.

A still further object of the invention, when operated in conjunction with a sulphuric acid concentrator, is the return of the hot denitrated acid to the sulphuric acid concentrator as a hot feed, thereby reducing the fuel consumption for concentration about 25% and increasing the capacity of the sulphuric acid concentrating unit.

Other objects will appear as the description proceeds.

These objects are accomplished by the following invention which not only utilizes the heat of dilution of the .sulphuric acid but permits the introduction of both acids into the process at a higher temperature than is practical with known existing processes. Accord.

ing to one embodiment of ourv invention the hot dehydrating agent may be added to either hot or cold nitric acid. This may be carried out either by placing the nitric acid in astill and then adding the dehydrating agent, or preferably by running the two liquids into the top of a packed column. This invention also contemplates preheating both the dehydrating agent and p the nitric acid. If a packed column is employed and the liquid allowed to flow through this column, a counter-current of air is used to conduct the vapors up the tower. The vapors of strong nitric acid emerge from the top or near the top of the tower, pass through a bleacher, and are condensed and delivered to a suitable receiver. The hot liquid acid mixture accumulating at the bottom of the dehydrating tower may be run directly to a denitrating tower of the conventional type in which steam is introduced intothe bottom in sufiicient quantity t'o denitrate the mixture. The nitric acid vapors forming in the denitrating tower may also be led to a bleacher and condenser.

By reason of thehigh temperature of the fsulphuric acid, it would be expected that the nitric acid would decompose and large "amounts of oxides of nitrogen would be produced, but this is not the case. On the contrary, only extremely small quantities of OX- ides of nitrogen are produced and these may be recovered together w1th any entrained nitric acid, which is not caught inthe condensers, by connecting the condensers to a suitably packed absorption tower.

vVithout'restricting it thereto, we describe the invention withparticularreference to the "apparatus illustrated in the accompanying drawing, and embodying it as it relates to tl 1-is*apparatus. The drawing shows a dia- 'grainma'tic side elevation of one assembly of the apparatus suitable for carrying out our "process. Thedehydrating tower,- 1, is con-' "lstr'ucted-of acid-proof material and is packed fwith some suitable acid-proof packing. The denitrating tower 2,'is of a construction similar' -toth'e dehydrating tower. Bleachers, 3

' and 4-,'are constructed of acid-proof material and are packed with acid-proof packing.

The condensers, 5 and 6, condense thevapors of each of the towers. 3..

The nitric acid is fed 'to the dehydrating column at 7 and may be preheated if so desired. The sulphuric acid 1 may be introduced into the dehydratingtower [and may be fed direct from the sulphuric concentrating unit, or may bGOthBIWlSB preheated or may be added'w'ithout heating.

' Air'may be introduced into the dehydrating tower at 9. Steam is introducedinto the botto-In of the denitrating column at 10. The

residual sulphuric acid which contains a small amount-of nitric acid is permitted to flow from the dehydrating tower at 11 directly into the denitrating column. p 'nitrated sulphuric acid flows from the denitrating column at 12. The strong nitric acid produced may be taken from the "apparatus The 'de at 13 "andjwe'ak nitricacid, whichmay'be used as such or which may be deliveredto the feed of the dehydrating tower, istaken from the apparatus at 14:. I

lVhen operating the apparatus as describedin the foregoing, at least 90% of the nitric-acid is obtained as strong nitric acid containing at least 94% acidity. The residual sulphuric acid leaving the denitrating tower will be approximately 78% strength sothat the amount of 100% sulphuric'acid required a's"93% to'95% strength to concentrate one pound of 100% nitric acid of62% strength will be approximately 3125 pounds; 'Although we have stated that thedehydrat- ,umn counter-current to steam the strength nitric acid obtained is over when no nitric is present in the efliue'nt from'thisdenitratin'g tower.

While we have described only one embodiment of our process as applied to a continuous tower operation it is within the scope of the invention to carry it out as a batch proc ess in a pot,'alth'ough a continuous process is preferable from the standpoint of economy and ease "of operation. It will be understood from" the foregoing that it is-possible to use the weak nitric acid from the denitratingfcolumn as -feed for the dehydrating column or this acid may be taken from the apparatus as weak acid. p I Y As many apparently widely different em 'bodim'ents of this invention may be made without departing from the spirit thereof; we do not intend to be limited tothe herin 'spec'ific embodiments except as indicated in the appended claims.

WVe claim: I

1. The process'ofproducing highly concentrated nitric acid which comprises adding from 2.7 to 6.0 parts of 100% sulphuric "acid of atleast strength, and'ait a'temperature of atleast 200 C. to'a'bout one part of="% nitric acid of at leasti50% strength soas to utilize the h-eat of the reaction to distill HNO vapors and condensing the strong'dis 'tillate. i i

2. The process'of'producinghighlyiconcentrated nitric acid which comprises 'mlxing a "dehydrating agent near itsboilingpoint with weak nitric acid so as to utiliie the heat of the reaction, and=passing the mixture'through a 'tower counter current to 'a gaseous-medium inert toboth acids.

8. The process of -claim 2 in-whichtheiiitric acid is near its boilingipoint.

4. The process'of c laiin 2'in-which thergas eous medium is air. 7 c

'5. The process of claim 2in which the dehydrating agent is sulphuric acid. J -6. The process of claim 2 in which the-dehydrating agent is sulphuric acid and the gaseous medium is air.

7. The process of p oducing *hi'ghly con- 'centrated nitric acid whichcomprises'mixing sulphuric acid near its "bOIlIII'g polnt with weak nitric acid near 1ts boiling point mamtaining the boiling temperature by the heat of the reaction and passing the mixture through a tower counter-current to a stream of air.

8. The process of producing highly concentrated nitric acid which comprises mixing a dehydrating agent near its boiling point with weak nitric acid near its boiling point, passing this mixture through a tower counter-current to a gaseous medium inert to both acids, recovering the residual hot dehydrating agent and subjecting it to a denitration with a minimum of steam, and adding the resulting condensed vapors to the dehydrating agent as it enters the tower in a continuous cycle.

9. The process of claim 8 in which the dehydrating agent is sulphuric acid.

10. The process of producing highly concentrated nitric acid which comprises introducing into a packed tower from 2.7 to 6.0 parts of sulphuric acid of 90 to 95% strength, maintained at a temperature above 200 (1., with one part of nitric acid of 50% to 66% strength, at a temperature above 20 C. so as to utilize the heat of the reaction to distill HNO vapors, recovering in the condensate over 90% of the nitric acid introduced as 94% or higher strength, passing the residual dehydrating agent through a tower counter-current to steam, and obtaining the remainder of the nitric acid as a condensate above 50% strength, with a denitrated residual of about sulphuric acid.

In testimony whereof we aflix our signatures.

IRVING J. COX. WILLIAM E. KIRST. CLIFFORD A. WVOODBURY. 

